Safety paper



Patented Aug. 10, 1937 SAFETY PAPER Clifford Paine and Jack Augustus Radley, Blackley, Manchester, England, assignors to Imperial Chemical Industries Limited, a corporation of Great Britain No Drawing. Application August 5, 1935, Serial No. 34,839. In Great Britain August 8, 1934 17 Claims. (Cl. 91-67-95) The present invention relates to the manufacture or preparation 01. improved safety papers, that is to say of papers mainly used for com mcrcial and legal documents, cheques, bank notes, bonds and the like, the paper being so treated as to prevent writing or printing thereon from being successfully tampered with.

It is known to employ chemically treated papersfor the prevention of tampering, the chemicals used for such treatment being selected so as to give visible stains in contact with inkeradicating agents. It has also been proposed to employ papers treated with fluorescent substances so that unauthorized alterations or the like, although not detectable in ordinary daylight, are made visible by inspecting the document under filtered ultra-violet light. In British specification 292,393 it is proposed to use for such treatment of paper a water-soluble fluorescent salt, namely the sodium salt of 2-naphtho1-3:6- disulphonic acid. The fluorescent substance described in the said specification has the disadvantage that the surface fluorescence produced by it in the paper is susceptible to destruction bywater alone. It is therefore possible for the cheque raiser or forger first to extract the fluorescent substance from the paper with water, make unauthorized alterations to the printed or written-document and thereafter restore the document to its normal appearance by re-treating it with a suitable fluorescent aqueous solution. To overcome this disadvantage a further improvement was proposed in British specification 417,488 whereby the fluorescent agent incorporated in or applied to the paper was to be a wa- I ter-insoluble compound, such as fluorine or acridine, in the form of a suspension.

There are, however, disadvantages in the use of water-insoluble fluorescent compounds for such purposes. In general it is necessary to use relativelylarge amounts of suspensions of such compounds in order to obtain the desired degree i of fluorescence unless extremely highly dispersed suspensions are used. Such dispersions are troublesome to make, liable to be unstable, or flocculated by other chemicals used in the preparation or finishing of paper so that uniform incorporation or surface application is made difficult. We have found that the disadvantages of the known processes can be overcome by the use of certain fluorescent compounds as hereinafter defined which are water-soluble but which,

. because of their strong aflinity for cellulose, are

extremely diflicult to remove from papers to which they have been applied.

According to the present invention, we incorporate into paper or apply superficially to it the free acid or a water-soluble salt of 4:4-dibenzoylaminostilbene-2:2'-disulphonic acid (see Stobbe and Mallison, Ber. der deutschen Chem. Gesellschaft, 1913, 46, 1235) or a substitution derivative thereof. As substitution derivatives we may use, for example, 4:4'-di-p-aminobenzoylaminostilbene-2:2'-disu1phonic acid, 4:4- di-o-chlorobenzoylaminostilbene-2 2'-disulphonic acid, 4:4-di-p-methylbenzoylaminostilbene- 2:2-disulphonic acid. The compounds for use according to the invention are readily made by reacting the appropriate aroyl halides with the sodium salt of 4:4'-diaminostilbene disulphonic acid in aqueous medium in the presence of a mineral acid binding agent, for example, sodium acetate or sodium carbonate.

The present invention may be carried into effect in a variety of ways. The said water-soluble fluorescent compounds as already mentioned have, although substantially colourless, a marked afflnity for cellulose, for example paper pulp, on which they may be dyed by the methods customary in applying direct dyestuffs to cotton. By virtue of these properties the said compounds may be incorporated into the texture or felting of a paper during manufacture. Thus paper pulp may be dyed with the said fluorescent compounds prior to the formation of a paper or pulp so dyed may be applied superficially to a layer of undyed pulp during the manufacture of a paper. Alternatively, a pre-iormed paper may be dyed by immersion in an aqueous solution of the said compounds. Where the invention is carried into effect by dyeing a pulp or paper it has been found that less than 0.5%, generally about 0.1% by weight of the fluorescent compound in relation to the weight of cellulose material to be dyed is sufiicient to obtain satisfactory fluorescence. In another embodiment of the invention an aqueous solution of the said fluorescent compounds may be applied superficially to paper by spraying, brushing or printing.

Papers treated according to the invention fiuoresce with a bluish hue when exposed to filtered ultraviolet radiations, whereas if subjected in parts to the action of ink bleaching chemicals such as calcium hypochlorite, permanganate, and oxalic acid or the like, or, provided a superficial coating of the stilbene derivative has been applied, if subjected to the action of an ink eraser, such localized treatment is immediately apparent under the ultraviolet lamp, the parts so treated ceasing to fluoresce and appearing dark against a bluish ground. Thus attempts to eradicate or alter writing on paper treated according to the invention are readily detected. Further, the fluorescent compounds used as indicators 01' such tampering are not readily removed from the safety papers by contact with water.

'The following examples illustrate but do not limit the invention. The parts are by weight.

Example 1 A solution of 0.1% strength of p:p'-diamino- 4 :4'-dibenzoyldiamino-stilbene 2 :2 disulphonic acid sodium salt is applied to the surface ofpaper at'a convenient part of a paper machine by means of a'water doctor or spray in the well known manner. The paper is dried as usual onthe drying rolls which follow\ the-said convenient part. If desired the stilbene sulphonic acid derivative can be applied to the paper pulp in the beater but since the whole body of thepaper will then be impregnated with the sulphonicacld, it will not be so sensitive to surface erasures, but will be equally sensitive to bleaching chemicals which penetrate through the paper.

Example 2' 7 applied to the surface oi'the paper by brushing,

dipping, spraying or printing. Drying is eil'ected in any convenient manner.

We claim:

1. The method of rendering paper and the like sensitive to ink eradicating chemicals which comprises incorporating therein a 4:4'-di-(benzoylamino) stilbene 2:2'-di-sulfonic acid compound.

2. The method of rendering paper and the like sensitive to ink eradicating chemicals which comprises incorporating therein an alkali metal salt of a 4:4'-di-(benzoylamino) stilbene 212'-di5lli1- ionic acid compound.

3. The method of rendering paper and the like sensitive to ink eradicating chemicals which comprises incorporating therein a 4 :4'-di-(aminobenzoylamino) stilbene 2 :2'-di-sulionic acid compound.

4. The method of rendering paper and the like sensitive to ink eradicating chemicals which comprises incorporating therein an alkali metal salt of a 4:4'-di-(benzoylamino) stilbene 2:2'-disultonic acid.

5. The method of rendering paper and the like sensitive to ink eradicating chemicals which cornprises incorporating therein an alkali metal salt oil a 4:4'-di-(aminobenzoylamino) stilbene 2:2- disulfonic acid.

6. The method of rendering paper and the like sensitive to ink eradicating chemicals which comprises incorporating therein an alkali metal salt of 4:4-di-(p-amino-benzoylamino) stilbene 2:2- disultonic acid.

'7. Paper and the like having incorporated therein a 4:4'-di-(benzoylamino) stilbene 2:2- di-suli'onic acid compound.

8. Paper and the like having incorporated therein an alkali metal salt of a 4:4'-di-(ben zoylamino) stilbene 2:2'-disulfonic acid compound.

9. Paper and the like having incorporated therein an alkali metal salt of a 4:4-di-(aminobenzoylamino) stilbene 2:2-disulfonic acid.

10. Paper and the like having incorporated therein an alkali metal salt of a 4:4'-di-(benzoylamino) stilbene 2:2-disulfonic acid.

11. Paper and the like having incorporated therein an alkali metal salt of 4:4'-di(p-aminobenzoylamino) stilbene 2:2-disulionic acid.

12. The method of rendering paper and the like sensitive to ink eradicating chemicals which comprises incorporating therein the sodium salt of 4 :4-di- (p-amino-benzoylamino) stilbene 2 2-disulionic acid.

13. The method of rendering paper and the like sensitive to ink eradicating chemicals which comprises incorporating therein 4:4'-di-o-chlorobenzoylamino stilbene 2:2'-disulfonic acid.

14. The method of rendering paper and the like sensitive to ink eradicating chemicals which comprises incorporating therein 4 4-di-p-methylbenzoylamino stilbene 2:2'-disulionic acid.

15. Paper and the like having incorporated therein the sodium salt of 4:4-di(p-amino-benzoylamino) stilbene 2:2-disulionic acid.

16. Paper and the like having incorporated therein 4:4'-di-o-chlorobenzoylamino stilbene 2:2'-disultonic acid.

17. Paper and the like having incorporated therein 4:4'-di-p-methylbenzoylamino stilbene 2:2 -disultonic acid.

CLIFFORD PAINE. JACK AUGUSTUS 

